For Assassin’s Creed 2, Plourde said the gameplay pillars were fighting, navigation, and social stealth.
I probably would have simplified this to fighting and navigation. Social stealth isn’t fully realized on its own so I’d call it a subset of navigation.
Regardless of that slightly differing matter of opinion, interviews like this one with lead designer Patrick Plourde are wonderfully educational to read. Like a post-mortem, his comments are insightful and honest, not colored and tweaked by PR (or, at least, not overly so). In the interview, he talks about the problems with combat, how they tied their mechanics and puzzles/levels together more tightly to always maintain a fun experience, and why they steer away from making the parkour more challenging — amongst other things (and yes, I realize this is an older interview. But I wanted to play the game before reading it, and then once I did play this post got lost in my drafts). The only sad thing is how short it is.